Since many electronic devices operating with direct current power operate at voltage of 3.3 V to 24 V, AC adapters, for example, have output voltages such as 5 V, 12 V, 19 V, and 24V. It is desirable that commercial transformers as well as transformers for switching power supplies have the optimum turn ratios suitable for their output voltages according to the principle of transformers which perform output voltage conversion based on the turn ratio. Accordingly, to design a power supply capable of varying an output voltage by two to three times (for example, a constant-current power supply), it is necessary to design the transformer and consider the withstand voltage of a primary side switching element and the withstand voltage of a secondary side switch element according to the maximum output voltage. This indicates that it is very difficult to design a single power supply or AC adapter capable of supporting devices having various operating voltages alone.
For this reason, in Japanese Patent Application Publication No. 2015-43643, a method of further providing a DC/DC converter, such as a step-down chopper regulator, at the DC output of an AC/DC power supply is used to configure a power supply capable of varying the output voltage largely.